the XP factor is a weighting system used to order guild (quantity/quality?)
Basically it is a muitiplier effect based on character levels.

As an example.. but not using the real factoring (as I'm too lazy to look at that code again just now)

Guild A has 90 characters.. all level 90

Guild B has 200 characters.. 10 level 90 and 190 level 1

guild A could have an XP factor of 90*90 or 8100
guild B could have an XP factor of 10*90 + 190*1 or 1090

Guild A is ranked higher in the list even though it is a smaller guild... it is a guild that has characters that have been more active.

If you were running the addon you could just use the chat command line to see the help info by typing /census
and in the list of commands you would see that you can get the info you are asking with the following
/CensusPlus who name _ Will display info that matches names or guilds.
where name is the guilds name.

If you just want to see what the website has.. then it is
Realm data/ guild info/ and enter the realm and guild names you are interested in.
if the info is available you will see a guild results box with the guild name underlined as a link.. click on the link and there you are.
Unless of course neither you nor anyone else has bothered to help this site by collecting the data and uploading it here.